1. Field of the Invention
The present invention relates to an acceleration sensor and, more particularly, to a MISFET (metal insulator semiconductor Field Effect Transistor) type semiconductor acceleration sensor having movable gate electrodes which can preferably be used for control systems for vehicles such as air bag control systems, ABS (antilock brake systems) and navigator systems, and for yaw rate sensing and pressure detection.
2. Related Art
Recently, there are increasing demands for more compact acceleration sensors (hereinafter referred to as G sensors) for automobiles at lower costs, in response to which single-chip G sensors have been prototyped using micromachining techniques. The Single-chip G sensors have a configuration wherein a mechanical structure is formed on a semiconductor substrate to directly process signals using an electrical circuit. An example of such a device is disclosed in U.S. Pat. No. 5,314,572 (Japanese PCT patent publication No. H4-504003, wherein a movable electrode serving as a weight is provided to form a differential capacitor, thus establishing a servo system and wherein an output signal is obtained by detecting acceleration acting on the movable electrode using a carrier wave.
It has been Droposed in Japanese Patent Application No. H5-322353 that a closed-loop controlled acceleration sensor which utilizes a capacitor or MISFET (Metal Insulator Semiconductor Field Effect Transistor) as a detection element and which includes a member for generating an electrostatic force to resist an acceleration force which acts on a movable portion to maintain the movable portion in position by canceling the displacement of the movable portion.
Further, in Japanese Patent Application No. H5-322348, it has also been proposed that an acceleration sensor having an arrangement wherein a movable portion is moved horizontally relative to the substrate to prevent it from being twisted and wherein acceleration is detected from changes in the amount of the current flowing through a MISFET.
According to the methods disclosed in the above U.S. Patent, however, the level of the output signal is low and the device can not be made compact, because the sensing device used is constructed as a differential capacitor having a movable electrode serving as a weight. Although the method according to the Japanese Patent Application No. H5-322353 employs a capacitor and includes a servo system provided for the purpose of detection, it suffers from a high noise level because high frequency signals such as a carrier wave are used. This has resulted in a problem that the above-described configurations can not provide sufficient signal strength when it is employed in sensors such as for ABS and yaw rate sensors which must be able to detect low levels of acceleration. There has been another problem in that the circuit for generating the carrier wave occupies about 40 to 50% of the entire integrated circuit making it difficult to construct a more compact sensor chip. In the method according to Japanese Patent Application No. H5-322348, although acceleration is detected from changes in the amount of current flowing through the MISFET, it is not possible to separately detect acceleration in only one dimension because acceleration is detected on a two-dimensional basis at the same time in directions vertical and horizontal with respect to the substrate.